1. Field of the Invention
The present invention relates to the manufacturing of semiconductor components. It more specifically aims at the implementation of a contact having an ohmic behavior over a lightly-doped P-type region.
2. Discussion of the Related Art
Conventionally, it is known in the field of semiconductors that, to make an ohmic contact between a semiconductive region and a metallization, the surface concentration of the semiconductive region must be relatively high (higher than 1019 atoms/cm3). This sets various problems for the implementation of such contacts, especially on P-type semiconductive regions. Here, the case where the P-type region to be contacted forms the anode of a one-way semiconductor component, that is, of a component blocking the flowing of a current when the P-type region is negatively biased, and enabling the flowing of a current when the P-type region is positively biased, is more specifically considered, this flowing of a positively biased current (forward biasing) being possibly submitted to the application of a control current or of a voltage exceeding a determined threshold.
To discuss the problem set, the structure of a high voltage rectifying diode will be considered in relation with FIGS. 1 and 2, that is, of a diode capable of withstanding a relatively high voltage in reverse biasing.
FIG. 1 shows an example of a first conventional high voltage diode structure. This diode includes an N-type central area 1 and includes on its upper surface side a P-type lightly-doped region 2 (Pxe2x88x92). PN junction 1-2 forms the junction of the considered diode. To ensure a sufficient breakdown voltage, the diode is of planar type, that is, the Pxe2x88x92 region is formed in a portion only of the upper surface of the central area-and is completely surrounded at its periphery by an N region corresponding to the central area.
To establish a contact on the rear surface of the diode, the lower surface of the central area includes a heavily-doped N+-type layer 3 coated with a cathode metallization 4. To take a contact on the upper surface of the diode, this upper surface is covered with a region 6 of an insulator, currently silicon oxide, which partially extends over region 2, a central region of layer 2 being cleared. In this central region, an overdoping is performed at the surface by forming a more heavily-doped P-type (P+) region 7 covered with an anode metallization 8.
FIG. 2 shows another conventional example of a high voltage diode. The cathode side including central area 1, overdoped region 3 and cathode metallization 4, is unchanged with respect to FIG. 1. On the anode side, anode metallization 8 rests upon a heavily-doped P-type region 10 surrounded with a P+-type ring 11. PN junction 1-10 this time forms the junction of the considered diode. As previously, an oxide layer 6 is present again at the surface, covering the periphery of region 10.
In the case of FIGS. 1 and 2, an ohmic contact has been made by forming a metallization directly on a heavily-doped P-type region. It is generally considered that the surface doping level of the P region must be higher than 5.1018 atoms/cm3. Further, the periphery of the heavily-doped P-type region is surrounded with a lightly-doped P-type region to improve the reverse breakdown voltage of the diode.
Of course, various alternatives of the structures illustrated in FIGS. 1 and 2 are known in the art. To improve the breakdown voltage, to best spread the space charge displayed by the reverse-biased junction, additional lightly-doped P-type guard rings, and/or field plates at the periphery of the Pxe2x88x92-type region (2, 11) and separated from the semiconductor by an oxide layer are, for example, used. N+-type regions, currently called stop layers, also block the potential lines likely to extend beyond the reverse-biased P+-type region (2, 11), thus avoiding the occurrence of a channel leakage current.
Further, to make an ohmic contact, a heavily-doped P-type region is always present, but this region is not necessarily directly covered with a single metallization. Currently, anode and cathode metallizations 4 to 8 are formed of an aluminum layer but other materials are also conventionally used (TiNiAu, Molybdenum). Pilings of metallic layers and/or of alloys, possibly incorporating silicon, are also provided. A silicide interface (IrSi, PtSi, NiSi, . . . ) formed by solid-solid chemical reaction between the silicide and the metal which has previously been sputtered may be provided. These are widely preferred to metal-semiconductor conventional contacts due to their high thermal and chemical stabilities.
The fact that the anode layer is formed of a heavily-doped P-type region in direct contact with a lightly-doped N-type region (FIG. 2) or in contact with a lightly-doped P-type region itself in contact with a lightly-doped N-type region (FIG. 1) has various disadvantages.
The first disadvantage is the necessity of providing a heavily-doped P-type region and a lightly-doped P-type region at the periphery. This requires the provision of several masking steps and complicates the manufacturing process.
Another disadvantage is the mere fact of having to provide a heavily-doped P-type region, which implies the requirement of providing a relatively high temperature anneal step of relatively long duration, for example higher than 1150xc2x0 C. for several hours, which takes up manufacturing time and is prejudicial to the crystalline quality of the silicon.
Eventually, and above all, in the case where it is desired to make a diode or another fast component, the P layer of which forms the anode, the presence of the heavily-doped P layer reduces the dynamic performance of the component including this P layer as an anode. Indeed, when the voltage applied to the diode is reversed, the diode must normally switch from the on state to the off state. But it is well known that a certain amount of time passes before the diode recovers its blocking ability. This time, currently called the recovery time, is that required for the evacuation of the carriers still stored in central area 1, which carriers have been previously injected by the anode region during the on-state operation. This carrier injection becomes higher as the doping of the P-type region increases. Thus, the rate of switching to the off state of the diode is limited by the presence of a heavily-doped P-type region. To overcome this disadvantage, the creation of defects in the central area and especially in the vicinity of the PN junction of the diode has been provided in prior art, for example by diff-using metals such as gold or platinum or by electron or proton bombarding or other. However, this solution causes other disadvantages and especially increases the forward voltage drop of the diode.
According to one aspect the present invention improves one-way components such as diodes or thyristors or even of components such as bipolar transistors meant to operate in a one-way mode, by reducing the recovery time, that is, the time of switching to the off state.
According to another aspect, the present invention provides a contacting structure that directly takes an ohmic contact on a lightly-doped P-type layer meant to be used as the anode of a one-way component.
More specifically, the present invention provides a contacting structure on a lightly-doped P-type region of a semiconductor component, this P-type region being meant to be positively biased during the on-state operation of said component, including on the P region a layer of platinum silicide, or of a metal silicide having with the P-type silicon a barrier height lower than or equal to that of the platinum silicide.
According to an embodiment of the present invention, the silicide is an iridium silicide.
According to an embodiment of the present invention, the P-type region has a surface doping level on the order of 1015 to 1016 atoms/cm3.
According to an embodiment of the present invention, the periphery of the lightly-doped P-type region is covered with an oxide layer.
According to an embodiment of the present invention, the P-type region is formed in a portion of the upper surface of an N-type region.
According to an embodiment of the present invention, the P region forms the anode of a diode.
According to an embodiment of the present invention, the P region forms the anode of a thyristor.
According to an embodiment of the present invention, the P region forms the gate of a thyristor.
According to an embodiment of the present invention, the P region forms the collector of a PNP-type bipolar transistor.
According to an embodiment of the present invention, the component, the P-type region of which forms an anode, is a vertical component.
The foregoing characteristics and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in connection with the accompanying drawings.